Many conventional sonar systems transmit a brief, single-frequency acoustic tone, often referred to as a ping, that is reflected from a passive target back to the originating source of the ping (e.g., a ship). The distance between the ship and the target corresponds to the time elapsed between transmission of the original ping and receipt of the reflected ping. The sonar system includes a transducer that converts between analog electrical signals and acoustical energy in the water. An analog signal processor employs a narrow bandpass filter and a threshold detector to identify the reflected ping as any signal having the single frequency and a magnitude greater than a preset threshold.
Such single-frequency sonar systems suffer from several disadvantages. For example, reliance on mere threshold detection of a signal is limited by the abundant background acoustic noise in water. Moreover, the analog signal processor is often unable to discern the ping reflected from the intended target from extraneous ping reflections from other targets such as an ocean bottom or other nearby vessels. Furthermore, the theoretical accuracy of threshold detection-type sonar ranging in a completely quiet environment is limited to the wavelength of the single frequency, with higher frequency (shorter wavelength) signals providing increased accuracy. Because higher-frequency acoustic signals suffer greater absorption in water, the use of high frequencies is limited to shorter ranges. In a completely quiet environment, a 10 kHz ping could theoretically have an accuracy of 15 cm for ranges of 5-10 km, and a 100 kHz ping could have an accuracy of 1.5 cm for a range of about 0.5 km.
As used in practical applications in ocean waters, however, threshold detection of single-frequency pings has an accuracy that is about 10 times worse than the accuracy expected in an idealized quiet environment. This degradation of accuracy stems from the use of signal filtering to distinguish the ping from background noise in the water. The usual frequency filtering process, together with environmental background noise, magnifies the uncertainty in the range measurement. Typically, long baseline sonar, which employs frequencies of about 10 kHz, has a practical measurement uncertainty of several meters. The magnified uncertainty introduced by the filtering and thresholding processes is basically a manifestation of a fundamental mathematical principle (i.e., the Heisenberg Uncertainty Principle) requiring that the product of uncertainties in frequency and position must exceed a fixed minimum value. Frequency-based signal filters having narrower bandwidths to avoid noise introduce larger ranging uncertainties.
To improve range resolution and to discriminate against clutter and extraneous signal reflections, some sonar systems transmit a brief multifrequency acoustic tone, which is often referred to as a chirp or pseudo-chirp. For example, U.S. Pat. No. 4,442,513 of Mead describes a sonar transceiver system that transmits four consecutive closely-spaced tones as a pseudo-chirp that is directed to and reflected from a passive target. Although it overcomes some of the limitations of conventional sonar, it suffers the inherent problems of frequency filtering and threshold detection described above. In addition, the Mead system necessarily uses a long wave train of over 0.5 sec duration and sonar signals of very high frequencies, thereby limiting the usefulness of the system in many applications. Specifically, the high frequency sonar signals limit the range of the system and the long wave train causes multipath signals from reflections to be difficult to interpret.